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Evidence from Accreted Seamounts for a Depleted Component in the Early Galapagos Plume

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Evidence from Accreted Seamounts for a Depleted Component in the Early Galapagos Plume Evidence from accreted seamounts for a depleted component in the early Galapagos plume David M. Buchs1, Kaj Hoernle2,3, Folkmar Hauff2, and Peter O. Baumgartner4 1School of Earth and Ocean Sciences, Cardiff University, Main Building, Cardiff CF10 3AT, UK 2GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstrasse 1-3, 24148 Kiel, Germany 3Christian-Albrechts University of Kiel, 24118 Kiel, Germany 4Institute of Earth Sciences, University of Lausanne, Bâtiment Géopolis, 1015 Lausanne, Switzerland ABSTRACT that ca. 90 Ma depleted komatiites from Gor- The existence of an intrinsic depleted component in mantle plumes has previously been gona Island (Kerr et al., 1995) and ca. 80 Ma proposed for several hotspots in the Pacific, Atlantic, and Indian Oceans. However, formation depleted basalts from Ocean Drilling Program of these depleted basalts is often associated with unusual tectonomagmatic processes such (ODP) Site 1001 in the Caribbean large igneous as plume-ridge interaction or multistage melting at plume initiation, where depleted basalts province (CLIP) (Kerr et al., 2009) provide addi- could reflect entrainment and melting of depleted upper mantle. Late Cretaceous to middle tional evidence for the existence of an intrinsic Eocene seamounts that accreted in Costa Rica and are part of the early Galapagos hotspot depleted component in the Galapagos hotspot. track provide new insights into the occurrence and nature of intrinsic depleted components. However, Site 1001 basalts formed on the top of The Paleocene (ca. 62 Ma) seamounts include unusually depleted basalts that erupted on the the CLIP by second-stage melting of a mantle Farallon plate far from a mid-ocean ridge. These basalts closely resemble Gorgona komatiites plume source that had already been melted to in terms of trace element and radiogenic isotope composition, suggesting formation from a form the base of the CLIP (Kerr et al., 2009), similar, refractory mantle source. We suggest that this source may be common to plumes, thus making a direct link with plume magmatism but is only rarely sampled due to excessive extents of melting required to extract melts from questionable. Gorgona komatiites are arguably the most refractory parts of a heterogeneous mantle plume. the best evidence for the existence of an intrinsic depleted plume component; however, these rocks INTRODUCTION also recycled, thus forming an intrinsic depleted are very unusual, without known equivalents in It is generally assumed that ocean-island component in mantle plumes (Kerr et al., 1995). the Mesozoic, and their exact provenance and basalts (OIBs) are derived from mantle plumes Basalts with depleted incompatible element link to the nearby Galapagos hotspot is unclear and that these melts commonly have more and isotopic compositions have been found at (Kerr and Tarney, 2005). enriched compositions than mid-oceanic ridge numerous hotspots, e.g., Iceland (Fitton et al., Due to ambiguities concerning the origins basalts (MORBs). The most widely accepted 1997; Thirlwall et al., 2004), Galapagos (White of depleted basalts formed in oceanic plateaus explanation for the enriched composition of et al., 1993), and Hawaii (Keller et al., 2000). or at hotspots where a plume interacts with a hotspot lavas is that plumes carry recycled mate- The depleted basalts are most common when mid-ocean ridge, it remains essential to provide rial such as altered oceanic crust and recycled the plume is located near a mid-ocean ridge. additional constraints on the possible existence sediment, which can be stored for tens of mil- The origin of the depleted component, however, of an intrinsic depleted component in mantle lions to billions of years in the mantle before remains controversial in most cases due to the plumes. Novel support for the existence of this returning to the surface, where the recycled similarity in composition between melts from component is provided here by new geochemi- material melts preferentially relative to depleted entrained depleted upper mantle and from a cal data from Late Cretaceous to middle Eocene upper mantle to form OIBs (Hofmann and depleted component intrinsic in the plume. Both accreted seamounts in the Osa Igneous Complex White, 1982). If the altered upper crust and sedi- will melt due to shallow upwelling and high- (Costa Rica), which formed at the early Galapa- ment of the oceanic lithosphere are recycled into degree melting of the plume beneath a ridge gos hotspot far from a mid-ocean ridge. the mantle, then it is likely that the lower, unal- potentially coupled with previous extraction of tered depleted crust and lithospheric mantle are enriched melts at depth. It has been proposed GEOLOGICAL BACKGROUND AND METHODS CENTRAL 100°W 90°W AMERICA 80°W The Osa Igneous Complex (OIC) is exposed CARIBBEAN PLATE on the Osa and Burica Peninsulas at the south- 10°N COCOS 10°N PLATE western edge of the CLIP (Fig. 1; Fig. DR1 East Pacific 1 Rise (EPR) in the GSA Data Repository ). The complex E Figure 1. Tectonic map of e includes an assemblage of Cretaceous to Eocene AT the central-eastern Pacific Coiba Ridge Ocean. Black areas in oceanic sequences predominantly composed of crust formed at the EPR Cocos Ridg Central America repre- sent accreted oceanic crust formed at the CNSC 1 CIFIC PL Malpeloe GSA Data Repository item 2016125, data tables, Cocos-Nazca Spreading Ridg sequences; square out- PA Figures DR1–DR3 (geological map and geochemical Center (CNSC) lines the location of Figure 0° 0° DR1 (see footnote 1). figures), and additional comments on tectonostratig- SOUTH raphy, analytical methods, data selection, and isotope NAZCA AMERICAN Galapagos data, is available online at www.geosociety.org/pubs PLATE Carnegie PLATE Islands / Hotspot Ridge /ft2016.htm, or on request from editing@geosociety .org or Documents Secretary, GSA, P.O. Box 9140, 100°W 90°W 80°W Boulder, CO 80301, USA. GEOLOGY, May 2016; v. 44; no. 5; p. 383–386 | Data Repository item 2016125 | doi:10.1130/G37618.1 | Published online 8 April 2016 GEOLOGY© 2016 The Authors.| Volume Gold 44 |Open Number Access: 5 | www.gsapubs.orgThis paper is published under the terms of the CC-BY license. 383 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/44/5/383/3549647/383.pdf by guest on 30 September 2021 3 Accreted basalts Figure 2. Selected geochemi- A B y Cocos and Group 1 (enriched) Carnegie Ridges Group 2 (intermediate) cal characteristics of igneous 1 accreted Group 3 (depleted) accreted OIBs OIBs rocks from the outer Osa Igne- N south Central ] Genovesa EPR off-axis EPR off-axis ous Complex (small diamonds American Icelandic plume arra Island seamounts Cocos and oceanic plateau seamounts Carnegie Ridges are samples from the inner Nb/Y Osa Igneous Complex). Data [Dy/Yb Gorgona komatiites sets used for comparison 1 0.1 Boninites Boninites include whole-rock and glass Site MORB MORB 1001 analyses from representative south Central America settings. EPR—East Pacific oceanic plateau Genovesa Gorgona Site Island Rise; MORB—mid-oceanic ridge komatiites 1001 0.5 0.01 basalt; accreted OIBs—accreted 0.21 6 11Zr/Y 0 [La/Sm]N ocean-island basalts in Central 10 America (for references, see C D the Data Repository [see foot- Cocos and Tholeiitic Alkaline y Carnegie Ridges OIB array note 1]; all selected samples y Genovesa Island accreted have MgO > 5.5 wt%). Site 1001 arra EPR off-axis 1 south Central American 1 seamounts OIBs volcanic arc arra oceanic plateau b MORB is from the Ocean Drilling Pro- b MORB-OIB gram. A: (Zr/Y) versus (Nb/Y) 2 N accreted BoniniteBoninites OIBs MORB array diagram (N—primitive mantle Th/Y Cocos and N TiO /Y 0.1 Genovesa EPR off-axis Carnegie Ridges normalized). B: Zr/Y versus Nb/Y Gorgona Island komatiites seamounts diagram (after Fitton et al., 1997). south Central American Gorgona Site oceanic plateau C: Nb/Yb versus Th/Yb diagram Site komatiites 1001 1001 (after Pearce, 2008). D: Nb/Yb MORB 0.01 0.1 versus TiO /Yb diagram (after 0.11 10 100 2 Nb/Yb 0.11Nb/Yb 10 100 Pearce, 2008). pillow and massive basalt flows with subordi- additionally indicated by loss on ignition values pelagic sedimentary rocks in igneous sequences nate gabbro (locally pegmatitic) and thin (<10 of 0.55–5.45 wt% (Table DR2). As a conse- of this group unambiguously support formation m thick) interbeds of pelagic and volcanicla- quence, the origin of the outer OIC is determined in a mid-ocean ridge or intraplate setting. The stic sedimentary rocks (e.g., Di Marco et al., based on a combination of immobile trace ele- trace element composition of group 1 closely 1995; Hauff et al., 2000; Hoernle et al., 2002; ment discrimination diagrams (Fitton et al., resembles that of seamount chains formed at Buchs et al., 2009). The complex has been sub- 1997; Pearce, 2008), radiogenic isotope ratios the Galapagos hotspot since the Miocene (Fig. divided into an inner OIC interpreted as a Late that are relatively insensitive to alteration, geo- 2). Isotope compositions are clearly enriched Cretaceous (ca. 85 Ma) oceanic plateau and an chemical comparison with a selection of possible compared to those of East Pacific Rise (EPR) outer OIC including several accreted seamounts volcanic analogues, field observations, and exist- MORBs (Fig. 3). This suggests that group 1 ranging in age between the Late Cretaceous (ca. ing regional constraints. New geochemical data could represent late-stage volcanic sequences 85 Ma) and middle Eocene (ca. 41 Ma) (Buchs from the inner OIC confirm an oceanic plateau et al., 2009). Tectonostratigraphic constraints origin; they have a composition indistinguish- define pre–late Eocene accretion ages for the able from contemporaneous Late Cretaceous 39.0 outer OIC (see the Data Repository). Here we oceanic plateau sequences observed elsewhere A Central America 38.5 oceanic plateau report new geochemical data that confirm an in south Central America (Hauff et al., 2000; Gorgona 38.0 komatiites oceanic plateau origin for the inner OIC; our Hoernle et al., 2002; Buchs et al., 2010).
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